A sensor for measuring a relative amount of an oxygen-included gas included in a gas mixture is described in Unexamined JP Patent Publication No. 2-122255. This sensor has two pump cells. These pump cells are positioned in series. In this sensor, a part of the oxygen-included gas (in particular, O2) included in the gas mixture is removed by applying a lower voltage to the upstream-side pump cell and the relative amount of the remaining oxygen-included gas (in particular, H2O and CO2) included in the gas mixture is measured by applying a high voltage to the downstream-side pump cell.